1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a backlight unit assembly using a light emitting diode array in a liquid crystal display device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for providing uniform white light.
2. Discussion of the Related Art
Recently, a liquid crystal display (LCD) device has been widely used in that the LCD device obtains a high quality picture image due to the improvement of liquid crystal and minute pixel processing technology as well as advantageous characteristics, such as thin profile, light weight, and low power consumption. Also, the application field of the LCD device has been continuously expanded.
Generally, an LCD module (herein after referred to as a liquid crystal module “LCM”) includes an LCD panel having lower and upper glass substrates, and liquid crystal injected between the upper and lower substrates, lower and upper polarizing plates on lower and upper surfaces of the LCD panel for polarizing light, and a backlight unit having a light source for providing light to the LCD panel, and a light guiding plate. The LCM displays a video signal input from the external source. Also, the LCD device includes a driving part for driving the LCM, and a system case.
FIG. 1 is an expanded view illustrating the related art LCM. As shown in FIG. 1, a related art LCM 10 includes a backlight unit 12 and an LCD panel 11. The backlight unit 12 and the LCD panel 11 are supported by a support main 13 and a top case 20. That is, a reflecting plate 12a, a light guiding plate 12b, a first diffusion or protection sheet 12c, a first prism sheet 12d, a second prism sheet 12e, and a second diffusion or protection sheet 12f are sequentially formed on the support main 13 of a plastic material, thereby forming the backlight 12. Then, the LCD panel 11 is deposited thereon. Meanwhile, the top case 20 of a metal material is connected to the upper side of the LCD panel 11, and the LCD panel 11 and the backlight unit 12 are supported by the support main 13. Also, an upper polarizing plate 11a is formed on the upper surface of the LCD panel 11, and a lower polarizing plate 11b is formed on the lower surface of the LCD panel 11.
Unlike a cathode ray tube (CRT) and a plasma display panel (PDP), the LCD panel 11 mounted on the LCD device requires an additional light source to emit light. That is, the LCD panel 11 controls the alignment of liquid crystal molecules by applying electrical fields. Thus, the LCD panel 11 additionally requires the backlight unit 12 for uniformly emitting light to the information display surface. Accordingly, the related art backlight unit uses one or more cold cathode fluorescent lamps (CCFL) as a light source.
In FIG. 2, a CCFL 25 is used as a light source of the backlight 2 in the related art LCM 10. As shown in FIG. 2, a receiving part 13a is provided in the support main (shown as 13 of FIG. 1) for receiving the CCFL 25, and the CCFL 25 is provided inside the receiving part 13a. The CCFL 25 is used as a light source. At this time, it is required to provide the light guiding plate 12b for guiding light to the LCD panel 11 since white light emitted from the CCFL 25 is diffused to all directions. For uniformly guiding the light emitted from the CCFL 25 to the LCD panel 11, the light guiding plate 12b gradually becomes thin as going to the opposite side of the end portion being in contact with the CCFL 25.
A fluorescent discharge tube is used for the CCFL 25 in order to utilize the penning effect, which is formed by injecting a hydrargyrum (Hg) gas containing argon (Ar) and neon (Ne) at a low pressure. Also, electrodes are formed at both ends of the fluorescent discharge tube, and the cathode is formed in a plate shape. When a voltage is applied thereto, electric charges inside the fluorescent discharge tube collide against the plate-shape cathode like a sputtering state, thereby generating secondary electrons. Accordingly, circumferential elements are excited by the secondary electrons, whereby plasma is generated. Also, the circumferential elements emit strong ultraviolet rays, and then the ultraviolet rays excite a fluorescent substance, thereby emitting visible rays.
In a backlight unit using the CCFL 25, the light source emits white light directly. However, if the backlight unit is used for a long time, it is difficult to maintain the emission of white light, thereby lowering a resolution and an efficiency in concentrating light. Also, it is hard to obtain the backlight unit having a high luminance due to size and capacity of the CCFL 25. Accordingly, it has been required to develop a light source having characteristics, such as high luminance, great resolution, low power consumption, and thin profile. In this respect, a light emitting diode (LED) has attracted great attention as the light source. The LED uses luminance when applying a voltage to a semiconductor. With the development of research and study for the LED, the LED begins to be applied to the LCD device as the light source.
Compared to the related art CCFL, the LED is miniaturized and has a long lifetime and low power consumption because it directly converts an electric energy to a light energy. Furthermore, the LED has a good luminous efficiency. Also, it is possible to obtain a high output power at a low current. In addition, the LED realizes a rapid response time, a high frequency modulation by pulse-operation, and an easy conversion of light output by controlling electric current.
FIG. 3 illustrates the process for generating white light in a backlight unit using an LED array as a light source. As shown in FIG. 3, the backlight unit includes an LED array 42 and a housing (not shown). The LED array 42 includes a plurality of red (R), green (G), and blue (B) LEDs 42a, 42b, and 42c for decreasing power consumption, and the housing surrounding the LED array 42 concentrates the light emitted from the LED array 42 at one direction. The LED array 42 including a plurality of LEDs respectively emits monochromatic light. This is because a multi-color LCD device includes a color filter as well as an LCD panel and a backlight. That is, the multi-color LCD device uses the backlight of a fluorescent lamp having three wavelengths as a light source. That is, the white light emitted from the backlight is divided into three of red, green, and blue colors in the color filter, and the divided colors are mixed again to display various colors.
Generally, the LCD device has characteristics, such as a viewing angle at which a viewer can watch a screen, resolution for realizing colors of red (R), green (G), and blue (B) through the color filter with the transmitted light, luminance for brightness of a picture image, and a residual image remaining on the screen after one picture image is displayed on the screen for a long time. In case of using the LED as a light source, compared to a case for directly emitting the white light, it is useful to emit red (R), green (G), and blue (B) light, and to generate the white light by mixing the red (R), green (G), and blue (B) light, for obtaining a better resolution when applying the LED to an LCD device.
The color of the light source is determined according to chromaticity coordinates of the Commission International De L'eclairage (C.I.E.). That is, tristimulus values ‘X’, ‘Y’, and ‘Z’ are calculated from a spectrum of a predetermined light source, and then x, y, and z chromaticity coordinates of red, green, and blue are calculated according to the conversion matrix. Subsequently, x and y values of the red, green, and blue are expressed as rectangular coordinates, so that a U-shaped spectral locus is drawn, which is called the CIE chromaticity diagram. The general light source has the chromaticity coordinates inside the U-shaped spectral locus. At this time, a triangle space of the red, green, and blue chromaticity coordinates becomes a resolution space. As the triangle space becomes large, a resolution ratio becomes high. The resolution depends on color purity and luminance. As the color purity and the luminance become high, the resolution increases. Herein, the tristimulus values ‘X’, ‘Y’, and ‘Z’ indicate weight of a color-matching function approaching to one spectrum. Especially, ‘Y’ is a stimulus value to the brightness.
Meanwhile, a color temperature means a temperature of the hue of the white color according to the color change of the light emitted by the temperature of a heat source. On the monitor, the color temperatures appear as 9300K, 6500K, and 5000K. That is, the white light has different luminous intensity distributions on each wavelength even though a viewer recognizes the white light as one hue. For obtaining a better resolution by transmitting the white light through the color filter, it is more useful to mix the red (R), green (G), and blue (B) light by using the LEDs emitting respectively red (R), green (G), and blue (B) light than to use the LEDs directly emitting the white light, whereby the white light is easily generated.
As the color temperature becomes close to 9000K, the hue of the white color contains a blue color. When the color temperature is 6500K, the hue of the white color contains a red color. When the color temperature is 5000K, a neutral hue is generated. The color temperature is obtained from the chromaticity coordinates (x, y) of the white color. As the color temperature becomes close to 9000K, it satisfies the European broadcasting union (EBU) standard.
In case of the aforementioned LCD device, a luminous spectrum of the backlight is coupled with the color-matching function and a transmission spectrum of the color filter to determine the tristimulus values at each wavelength of the visible ray region. That is, in order to obtain the various colors, it is required to control a correlation between the backlight/color filter and the tristimulus values. In other words, the luminous spectrum of the backlight has to be controlled to optimize the resolution and the color temperature, and the transmission spectrum of the color filter has to be controlled to optimize luminosity.
To obtain the white color, it is necessary to simultaneously use the red (R), green (G), and blue (B) LEDs, thereby causing many problems in applications. Especially, in case of using the red (R), green (G), and blue (B) LEDs simultaneously, different colors emitted from the respective LEDs have to be mixed to generate the white color. In this case, it is required to emit the red (R), green (G), and blue (B) light from the LEDs having a predetermined luminous intensity.
Accordingly, the backlight unit using the LED array as a light source includes a light source part having a plurality of red (R), green (G), and blue (B) LEDs 42, and a light guiding plate 12b for mixing the light emitted from the light source part, and uniformly dispersing the white light to an LCD panel (not shown). When the LEDs 42 are used as light source, the respective LEDs 42 emit the red (R), green (G), and blue (B) light to generate the white light. However, in region “a” of the light guiding plate 12b, there is a portion 60 where the light emitted from the respective LEDs 42 is not mixed, whereby the white light is not generated in the portion 60. In region “b” of the light guiding plate 12b, the red (R), green (G), and blue (B) light emitted from the LEDs 42 is mixed to generate the white light. Thus, in the aforementioned backlight unit, it is required to transmit the white light generated in region “b” of the light guiding plate 12b to the LCD panel.
However, the backlight unit of the LCD device according to the related art has the following disadvantages.
In the related art backlight unit using the LED array as a light source, the light leaks at the side portion of the LED array. Even though the housing is provided in the related art backlight unit, it is difficult to concentrate the light to one direction, thereby lowering the luminance.
Also, in case the LED array is used as a light source, the red (R), green (G), and blue (B) light is respectively emitted from the LEDs of the LED array. In this case, the light emitted from the LED positioned at the side portion of the LED array leaks to the external, so that it is difficult to generate uniform white light, thereby lowering resolution of the LCD device.